I’m starting on the roofs of both and then using my cash to get the house rent ready.

Pitched or Flat Roof ?

Did you replace the roof (looks a little old in the pics) or any mechanicals, plumbing, etc.?

Im doing most of the renovation myself, and the HELOC just barely covered the down payment and closing costs.

The home can use a new roof, pressure wash, outside paint and some landscaping. 

Avoid deals where it requires a new roof or foundation, they are most costly.

I'll cover his payments and give him his $350k asking price.

Maybe your expenses will get covered by the other rents, but unlikely you’ll have excess cash.

The current DOM for those particular zip codes (which covers everything from Texas City to Clear Lake to Pasadena and everything surrounding) still shows that we are still in a strong seller's market. 

I would do this if it meant saving enough on purchase price to justify the seasoning period, the interest rate risk, and I still had adequate reserves to cover costs and expenses.However if you aren't able to see real savings on our has price then you should finance it and use Heloc for down payment only.